1. Field of the invention
The present invention relates to an error detecting unit of a rotational detector such as an encoder, and particular to an error detecting unit of a rotational detector, by which an error of the rotational detector can be securely detected without fail.
2. Description of the Prior Art
Generally, a rotational detector such as an encoder is attached to a rotary body (for instance, a motor), and it can output two kinds of pulse signals (A and B) having phase difference (for instance 90 degrees) and equal frequency according to rotation of the rotary body.
FIG. 4 and FIG. 5 illustrate one of the examples of the pulse signals A and B as utilizing a time chart, and these Figures respectively show the state of pulse signals when the motor rotates counterclockwise (hereinafter called "CCW direction") or clockwise (hereinafter called "CW direction").
Generally, the actual direction (CCW direction or CW direction) of rotation of the motor can be distinguished by difference of the level (Low and High) of the pulse signal B at the times (k), (l) in FIG. 4 and at the times (m), (n) in FIG. 5 when the first transition of the pulse signal A occurs.
Usually, the direction by a command of the direction of rotation of the motor is coincident with the actual direction of rotation. However, in the case of the system in which the motor is given reverse driving force by changing a command of direction of rotation in changing the direction of rotation of the motor or braking, the direction of rotation by a command of the direction of rotation in changing the direction of rotation or braking the motor is reverse of the actual direction of rotation thereof.
It has been known that, as an error detecting method of the rotational detector, such a method that can determine that the rotational detector is out of order when the direction by a command of the direction of rotation of the motor is not coincident with the actual direction of rotation of the motor at other time than in changing the direction of rotation or braking as shown in the above.
Hereupon, there are many cases that the error of the rotational detector is a short-circuit, breakage of wire, etc. of a signal transmission channel. In these cases, the above pulse signal (A or B) remains as it is at low level or high level.
FIG. 6, using a time chart, shows an example that the pulse signal B is kept at low level as one of the examples of error.
In this case, it is determined that the actual direction of rotation of the motor is usually CCW direction.
Therefore, according to the conventional error detecting method for the rotational detector, in the case that the direction by the command of direction of rotation is CW direction, it is possible to detect the above error. However, in the case that the direction by the command of direction of rotation is CCW direction, such an error as shown in the above can not be detected. For this reason, an error of the rotational detector could not be reliably detected by the conventional error detecting method as shown in the above.